Shearing is made much use of for manufacture of the metal members used in automobiles, household electrical appliances, building structures, ships, bridges, construction machinery, various plants, penstocks, etc. FIGS. 1 and 2 schematically show modes of this shearing. FIG. 1 schematically shows the mode of shearing for forming a hole in a blank, while FIG. 2 schematically shows the mode of shearing for forming an open cross-section in a blank.
In the shearing shown in FIG. 1, a blank 10 is placed on a die 40 (below, also referred to as a “first blank”) and a punch 90 is pushed into the blank 10 in a sheet thickness direction 90a to form a hole in the blank 10. In the shearing shown in FIG. 2, a blank 10 is placed on a die 40 and similarly a punch 90 is pushed into the blank 10 in the sheet thickness direction 90a to form an open cross-section in the blank 10.
Referring to FIG. 3 and FIG. 4, the shape and shaping mechanism of the sheared edge formed by the mode shown in FIG. 1 or FIG. 2 are shown. FIG. 3 shows a cross-sectional schematic view of a sheared edge 19 of a worked material 12 formed by shearing, while FIG. 4 shows a cross-sectional schematic view of shearing using a punch 90, die 40, and holder 50 for obtaining a punched out material 11 and worked material 12. The sheared edges of the punched out material 11 and worked material 12, usually, as shown in FIGS. 3 and 4, are comprised of shear droops 14, 14′, burnished surfaces 15, 15′, fracture surfaces 16, 16′, and burrs 17, 17′. The shear droop 14 is formed at the punch side surface 18a of the sheared edge by the blank 10 being pushed in by the punch 90. As shown in FIGS. 1, 2, and 4, a clearance CL is provided between the punch 90 and die 40 so that the punch 90 and die 40 will not contact when the punch is pushed in the sheet thickness direction 90a. The clearance CL is necessary for securing a certain extent of distance for obtaining a contact margin of the punch 90 and die 40. When the punch 90 pushes the blank 10 in the sheet thickness direction 90a for shearing, the blank 10 is drawn into the clearance CL of the punch 90 and the die 40 whereby the blank 10 is locally pulled against and a burnished surface 15 is formed. The fracture surface 16 is formed by the blank 10 drawn into the clearance CL of the punch 90 and the die 40 being fractured. A burr 17 is formed at the die side surface 18b of the sheared edge when the blank 10 drawn into the clearance CL of the punch 90 and die 40 fractures and separates.
A sheared edge generally suffers from the problem of being inferior in surface properties, lower in fatigue strength, or lower in hydrogen embrittlement resistance compared with a worked surface formed by machining.
Numerous techniques have been proposed for solving the problem of a sheared edge. These techniques generally can be divided into ones which specially devise the structures of the punch and die to try to improve the surface perpendicularity and surface properties (fatigue strength etc.) of the sheared edge (for example, see PTLs 1 to 3) and ones which shave, coin, or otherwise process the sheared edge to try to improve the surface perpendicularity and surface properties (fatigue strength, hydrogen embrittlement resistance, etc.) (for example, see PTLs 4 to 6).
However, in the techniques specially devising the structures of the punch and die, there is a limit to the improvement of the surface perpendicularity and surface properties of the sheared edge. Further, in the techniques of processing the sheared edge, the productivity falls and the manufacturing costs rise by the amount of the increase of one step. Further, when working a high strength material, a tool easily suffers from wear, chipping, or other damage.
PTL 7 discloses a working method and working apparatus stacking shearing mechanisms of punches and dies and successively shearing metal sheets placed on the dies by pushing down the punches. In the working method and working apparatus of PTL 7, the productivity is improved and the manufacturing costs fall, but it is difficult to raise the surface perpendicularity and surface properties of the sheared edge of the worked material and the punch and/or die is damaged when shearing a high strength material.
NPTL 1 discloses post processing a punched out material blanked into a predetermined shape during which placing a blade at the die side and using a punch larger than the die to shave doubly stacked blanks in an overlaid blanking and shaving method. However, when blanking into a predetermined shape, the punch or die is damaged. On top of this, when shaving, the die with the blade may be damaged.
In the final analysis, in the prior art, it is difficult to shear a material while securing a sheared edge with excellent surface perpendicularity and surface properties and suppressing tool wear and damage.